(1) Field of the invention
This invention relates to the preservation of moist plant-derived raw materials. More particularly the invention relates to the preservation of wood substance, inherent tall oil and brightness in wood chips during stockpiling.
(2) Description of Prior Art
Outside storage of wood chips was introduced in the early 1950's because of the need to stockpile chips produced from sawmill and veneer plant residues. When the economic advantages of handling wood as chips became apparent, many pulpmills begain stockpiling chips instead of storing wood as logs. Although it is cheaper and easier to handle and store wood in the form of chips, increased wood deterioration occurs during outside chip storage. Wood chips lose more wood substance, tall oil and brightness during storage than do logs.
One principle difference between piled log and piled chip storage is that a significant amount of heat is evolved in piled chips. Center temperatures in chip piles frequently reach and remain near 150.degree. F., thus resulting in deterioration of the material.
Heat production is caused by enzymatic respiration of living wood cells, respiration of rapidly multiplying populations of bacteria and fungi and direct chemical oxidation. Prevention of the heat release from bacteria, fungi, and living wood cells would stop the initial temperature rise and prevent the direct chemical reactions from releasing any significant amount of heat.
Chemical treatments for preventing chip deterioration should stop or reduce the initial heat evolution by inhibiting the respiration of living wood cells and the respiration of all present bacteria and fungi. A treatment that kills the wood cells, bacteria and fungi would be highly effective. The treatment must remain effective for long periods of time since reinnoculation of the chips by airborne microorganisms will be constantly taking place. Any chemical treatment should be effective for a considerable length of time and its cost should be less than the losses incurred from chip deterioration. It should be compatible with the pulping process, should not cause pollution and should not be hazardous to personnel who handle and apply it.
One method found to be effective in reducing wood substance losses in stored wood chips by limiting the effect of wood-destroying fungi, is described in U.S. Pat. No. 3,646,196. This method, using kraft green liquor, inherently produces a large reduction in wood brightness and has little favorable effect on tall oil retention.
Another method uses sodium bisulfite and various phenols and is described in U.S. Pat. No. 4,045,554. These treatments are effective against all three major effects of deterioration, i.e., wood substance, tall oil, and brightness losses. One disadvantage of these treatments is that the sodium bisulfite component is very corrosive to steel and most types of stainless steel. Another disadvantage is that the most effective phenol when used together with sodium bisulfite is 2,4 dinitrophenol which is very toxic to humans and to animals.
Formaldehyde has been used as a preservative for moist plant and animal-derived materials, and is effective for only a few weeks according to the inventor's tests. Phenolic compounds such as para-nitrophenol have also been used to preserve moist plant derived materials. However, they are ineffective in reducing heat evolution in stored wood chips.